The present invention relates to antiskid brake control systems for carrying out control of the braking fluid pressure to prevent wheel lock during braking, i.e. antiskid brake control so called, particularly, at low vehicle speed.
The antiskid brake control systems serve to control the wheel-cylinder pressure or braking fluid pressure to prevent wheel lock during braking for stabilization of the behavior of a vehicle body. Generally, the antiskid brake control systems are constructed to carry out pressure increasing control for increasing the braking fluid pressure, pressure reducing control for reducing the braking fluid pressure, pressure holding control for holding the braking fluid pressure, pressure gradually increasing control for gradually increasing the braking fluid pressure, etc. as appropriate in accordance with the relative relationship between the vehicle-body speed and the wheel speed, i.e. the slip ratio so called.
During braking, repeated involved occurrence of nosedive and squat produces load variations in front and rear wheels. The load variations can occur, particularly, at the initial stage of braking, and stabilize thereafter comparatively. Such load variations affect antiskid brake control seriously. Specifically, even if the braking fluid pressure is controlled at a value appropriate to, e.g. increased wheel load, subsequent load variations bring a reduction in wheel load to make the braking fluid pressure excessive, heightening the lock tendency of the wheels. Particularly, when load variations occur in the low-speed area where the wheel speed is low per se, zero wheel speed, i.e. wheel lock, is apt to be produced.
If the wheels lock due to load variations or the like, elimination of wheel lock during a short period of time requires a quick pressure reduction. However, frequent execution of quick pressure reduction leads to increased amount of brake fluid discharged from a wheel cylinder to a reservoir. Since further pressure reduction cannot be carried out if the amount of discharged brake fluid reaches the capacity of the reservoir, brake fluid should quickly be returned to a brake circuit by using a large-capacity pump. The use of a large-capacity pump results in an increase in manufacturing cost and size of the system.
Wheel lock is apt to occur in the low-speed area as described above, whereas if a quick pressure reduction is carried out in the low-speed area, a driver will have an unfavorable G lowering feel so called or feel that deceleration is decreased suddenly.
With the structure wherein the amount of pressure reduction is set in accordance with wheel acceleration, occurrence with wheel lock may cause zero wheel acceleration, resulting in impossible achievement of appropriate amount of pressure reduction.
It is, therefore, an object of the present invention to provide antiskid brake control systems which contribute to a reduction in manufacturing cost and size of the system and to an enhancement in a drive feel, particularly, in the low-speed area.
The present invention provides generally an antiskid brake control system, which comprises: a sensor which senses a wheel speed; a braking device comprising a valve which allows increase and reduction in a fluid pressure in a wheel cylinder; and an electronic control unit (ECU) connected to the sensor and the braking device, the ECU estimating a wheel slip in accordance with the sensed wheel speed and controlling the valve for braking operation in accordance with the estimated wheel slip for an antiskid brake control, the fluid pressure being reduced when the wheel slip is enlarged and being increased when the wheel slip is lowered, the ECU performing a quick pressure reduction prohibiting control to restrain a quick pressure reduction having a greater reduction amount than a predetermined reduction amount when a vehicle-body speed is smaller than a predetermined quick pressure reduction prohibiting speed.